The goal of a paper path system in a typical xerographic printing system is to transport media from a feeding unit in synchronism with a moving image bearing photoreceptor surface. The media necessarily must arrive at the transfer zone at a given time and with a given velocity to match the velocity of the image bearing photoreceptor surface. Traditional media handling systems have relied on the use of expensive and precisely manufactured actuators (such as roller transports) for moving media such as paper and transparencies with very little or no feedback control involved. These systems typically do not perform very well when subjected to handling a wide range of media as well as exhibit problems with maintaining accuracy and reliability at high speeds. The present invention uses a more control-centric design of media handling systems that takes advantage of the diamatic decrease in chip cost and moves away from parts requiring high tolerance. It does so by embedding more controls in the system and trimming the overall cost by reducing the cost of hardware. The invention also enables significantly better performance by being able to handle a wider range of media at higher speeds through effective use of modern control strategies. Also prior art systems are often open loop systems with the media running at a specific speed and position adjustment being made at a transfer registration station just prior to transfer. A difficulty with such systems is the often erratic and abrupt adjustments that must be made at the registration station due to the unpredictability of photoreceptor and media drives and the uncertainty of the position of the image on the photoreceptor. With little time and space for adjustment, the correction can be erratic. This is particularly true in higher speed, higher volume machines.
It is known in the prior art, for example, U.S. Pat. Nos. 5,328,168 and 5,257,070 to selectively activate copy sheet drives after a machine jam in order to position copy sheets for favorable jam clearance including the steps of maintaining a predetermined interdocument space between copy sheets and systematically purging copy sheets from zones of the paper path in a predetermined order. A difficulty with these prior art systems, however, is the restriction of the systems to jam recovery. It would be desirable, therefore, to provide a relatively smooth and more accurate adjustment technique over the entire paper path to synchronize the arrival of copy sheets and images on a photoreceptor at an image transfer station.
It is an object of the present invention, therefore, to provide a multi-layered hybrid hierarchical control architecture for media handling. It is another object of the present invention to provide a combination of modular, discrete and continuous controllers that interact with copy sheets as the copy sheets move along a paper path. It is still another object of the present invention to provide a discrete controller to plan distance-time trajectories for media on a media path and to provide continuous controllers for keeping the media on the respective trajectories using multi-layered architecture. Further advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.